The illumination required for motion pictures and television has changed drastically over the years. The first motion pictures were shot in daylight. The mercury arc and open carbon arc's were the first artificial light sources used. The introduction of the incandescent lamp resulted in film being developed that was compatible with this warm source as opposed to the cool daylight quality of previous sources. The development of this film lead to the development of a carbon arc with a warm, incandescent color quality rather than just the cool, daylight color. The development of xenon lamps in the 1950's and 60's with their near perfect daylight color resulted in their being applied to film and TV illumination.
As TV broadcasting developed during the 50's and 60's, it used the lighting systems then being used for film that included daylight, carbon arcs, xenon, and incandescent. In the 60's, the incandescent lamp was improved with the introduction of the tungsten-halogen lamp. The desire for dimming prominent in TV studios, but not film studios, led to the predominant use of the tungsten-halogen incandescent lamp for TV studio applications. This was necessary since carbon arc's and xenon lamps are not easily dimmed.
During the 70's the development of fast film and easily portable TV cameras led to an increased desire to shoot pictures outside the studios. Since daylight was often present, the desire was for the development of a light source that was easily portable and had a daylight color. The carbon arc and xenon lamps required direct current which required a separate generator. This limited the ease of their use and portability. Since the available power was often limited, the source should also have as high a luminous efficacy as possible from a small source for optical control. This resulted in the development of the HMI metal halide lamp which has a near perfect daylight color, compact source and as much as 100 lumens per watt. Compared to the similar appearing compact xenon lamp, the HMI metal halide lamp has up to 21/2 times the efficacy (40 vs 100 lpw).
The HMI lamp has been developed in wattages ranging from 200 to 6000 watts and operated off any alternating current available source of power. A recent development in this lamp type has resulted in an incandescent quality metal halide lamp, whose luminous efficacy is about 3 times that of an incandescent lamp. Other metal halide lamps with different configurations have also been developed, termed CSI and CID. These lamps are available in open configurations as well as sealed into a PAR 64 reflector lamp which replaces a PAR 64 1000 watt tungsten-halogen lamp.
The construction of the HMI metal halide lamp involves a central arc chamber of thick quartz with two long legs extending out along the lamp axis. The long legs are necessary to accomplish a tight seal between the quartz envelope and metal electrical lead. The lamps, especially the higher wattages, have a high current which produces high temperatures in these seals which must be limited to about 300 degrees C. at the point where electrical contact is made. The temperature in the arc chamber, on the other hand, must be at least 600 degrees C. to obtain the desired color quality.
The CSI and CID lamp uses a press seal where metal ribbons are sealed into quartz. A 300 degrees C. limit at the lower end of the press is again desirable, especially when the lamp is not sealed into an inert atmosphere inside a PAR 64 lamp. Their quartz envelope is not as thick as the HMI lamp envelope, but a 600 degrees C. bulb wall temperature is present.
To help reduce the seal temperatures of all of these lamps, the electrical contact elements are often massive heat sinks with cooling fins. Forced cooling could help, but it would be an added noise producing consideration since the lights and microphones are often in close proximity. The thermal characteristics of the luminaires and sockets are therefore an important consideration.
The most common luminaire used for film and TV illumination is the fresnel lens spotlight of the type seen in the Richardson U.S. Pat. No. 2,057,278. Such luminaires were developed for the incandescent lamp and used a spherical reflector to reimage the filament back through the openings in the filament to make the beam pattern more uniform and more efficient. When a carbon arc or compact xenon arc are used with a fresnel lens, there is no gap in the arc to fill by imaging the arc onto itself with a spherical reflector. These direct current arcs have one large electrode where a point of high brightness exists and a directional pattern of light emits from this point. When a fresnel lens is placed in front of this directional beam, it collects and redirects this beam into the desired spotlight pattern and no auxiliary reflector is needed; see U.S. Pat. No. 1,364,866.
Over the years many luminaires have been developed with carbon arc, mercury arc, xenon arc and incandescent light sources. Some such patented systems have included Arrousez, U.S. Pat. No. 1,845,214; Taillon, U.S. Pat. No. 3,379,868; Richardson, U.S. Pat. No. 2,057,278 and German Utility Model No. 1,744,824 dated May 15, 1957. The most popular luminaires for film and TV lighting have been the Brute Arc which uses the open carbon arc source and a fresnel lens for a high intensity spotlight for both outside, daylight quality light as well as inside, incandescent colored light. For inside use, the incandescent fresnel lens spotlight has been the primary workhorse of the industry.
The creation of the highly efficient, metal halide sources has presented the desire for their use in standard film and TV type luminaires. The creation of these lamps has been accompanied by the creation of special luminaires, but the desire to incorporate the lamps into a fresnel lens spotlight has been a primary desire of the industry. The lack of one bright spot and a directional quality of the source has made any use of these new metal halide lamps in the standard Brute Arc to be ineffective.
The direct replacement of the incandescent lamp with one of the new metals halide lamps produces less than adequate results. Such direct replacement would find the spherical reflector reimaging the arc onto itself without the filament gaps through which the energy passes the reimaging only becomes energy that is absorbed resulting in increased bulb heating. The arc configuration is such that the resultant beam is oval rather than circular, as is the case with the standard fresnel luminaires.
Some fresnel lens luminaires designed for the metal halide lamps change the light source/reflector orientation to image the source around the bulb which reduces the bulb heating and can produce an adequate beam pattern. Improvements are still desired and the potential for retrofitting existing luminaires without bulb temperature problems has been a continued desire of the industry. A method of using the metal halide lamps in the standard luminaires without major reconstruction, bulb overheating, efficiency reduction, light quality sacrifice or other compromises has presented a challenge.
The standard fresnel lens luminaires are not highly efficient. Typically, only about 10 percent of the lamp's energy is collected into the beam of the fresnel luminaire at full spot and 30 percent at full flood. One development of a more efficient fresnel lens spotlight can be seen in the Levin/Lemons U.S. Pat. No. 3,428,800. The new metal halide lamp source size and construction makes this type of system impractical. The only efficient metal halide lamp systems developed to date have been lamp/reflector combinations which do not include the use of a fresnel lens. These systems can be relatively small, light weight, highly efficient and meet all other criteria except the desire for the industry standard fresnel lens type luminaire.